Real time automation systems are sensitive to loss of data. Data can be lost in such systems for several reasons, for instance due to errors on links or network devices, external electrical disturbances, radio disturbances or resource problems in network devices. Any such failure within a real-time control system supervising, for instance, an electrical system, may lead to severe consequences.
In order to reduce the risk of communication losses redundancy may be added. In a layer-2 Ethernet network with ring topology all devices are connected by two ports to the Ethernet network. A media redundancy protocol manages reconfiguration if a link is broken. A drawback with this method is that reconfiguration after failure takes time and any lost data has to be retransmitted, which entails delay. Another problem is that a ring is logically a single Ethernet network with little protection against disturbances such as intermittent link errors or broadcast storms.
Another Layer-2 method uses a parallel Ethernet structure as defined in the International Electrotechnical Commission (IEC) 62439-3 (Parallel Redundancy Protocol, PRP). A PRP network implements redundancy functions in the communicating end nodes, which are each attached to two different and completely separated Ethernet networks; the two Ethernets have no direct connection. As long as one of the Ethernet networks is operational the frame sent from a source node reaches the destination node, which receives only the first message and discards the duplicate message. Hence, no data is lost even if one of the networks would fail and the PRP is a good option for use within an Internet Protocol (IP) subnetwork such as Ethernet.
US2006/0256768 describes a routing method in a communication network minimizing overlap between two paths in the network over which an original packet and its duplicate are sent.
Although PRP provides high reliability, it is limited to work only within the IP subnetwork. The mentioned real-time automation systems may comprise applications which are highly sensitive to loss of data also from devices of remote networks and even a shorter disturbance in the remaining functioning IP subnetwork may then have critical effects.